The present invention relates to automotive structural members for automobiles and trucks. More particularly, the present invention relates to a method of manufacturing original equipment and after-market automotive structural members such as vehicle pillars, sub-frames, cross beams, frame rails, frame brackets, roof rails, seat frames, door beams, bumper beams, control arms, wheels, instrument panel reinforcements, running boards, roll-bars, tow hooks, bumper hitches, or roof racks.
It is preferred that automotive structural members be lightweight to provide improved fuel economy, and of a sufficient strength and durability to meet automotive safety requirements. In addition, automotive structural members must be able to contend with harsh environmental conditions, and thus must be corrosion resistant.
In cost-sensitive applications such as automobiles, conventional engineering materials force a trade-off between cost and fuel efficiency, safety, and performance. Consequently, the typical vehicle tends to have a frame that is both too heavy and too weak. A heavy frame requires a more powerful engine, which leads to higher fuel consumption and higher emissions. The more powerful propulsion system is itself more expensive to build, uses more material, requires more energy to produce and leads to more emissions related to its manufacture. Conversely, a lightweight weak frame compromises the durability of the vehicle and the safety of its occupants.
Unfortunately, present day automotive structural members are still undesirably heavy and expensive to manufacture. For example, the automotive industry has recently introduced new alloys into automotive structures to improve hardness in an effort to reduce weight by reducing material. Furthermore, complicated and expensive coatings and heat treatments has been introduced to improve the characteristics of corrosion resistance, hardness, tensile strength, and toughness. Examples include efforts described in U.S. patent application No. 2006/0130940 which describes a nickel coating process for automotive components, and U.S. Pat. No. 6,475,307 which describes a method of manufacturing automotive components of stainless maraging steel. Several attempts have also been made to selectively harden only portions of automotive structural members, such as described in U.S. Pat. No. 5,868,456 and U.S. Patent Application No. 2003/0025341.
Unfortunately, all of the aforementioned attempts at manufacturing structural automotive components still suffer from various drawbacks. For example, prior manufacturing processes are either too expensive or produce automotive structural members having characteristics which are less than desirable such as a lack of hardness, durability, corrosion resistance, etc. As graphically depicted in FIG. 1, structural materials are currently available in a broad range of strength-to-weight ratios, or specific strengths, but the costs of these materials generally increase disproportionately to their specific strengths. Carbon composites and titanium, for example, while being perhaps ten times stronger than mild steel for a given weight, are typically more than fifty times more expensive when used to bear a given load. Consequently, such high performance materials are typically used only in on small items or in applications where the high cost is justified, such as in aircraft.
Conventionally, automotive structural members are manufactured from non-air hardenable steels. A rare exception of this is boron steel which provides high strength but it is not particularly corrosion resistant. Furthermore, the use of boron steel for automotive structural members typically requires implementing unwanted and expensive manufacturing steps to remove scale resulting from the hot-stamping hardening process.
An example of a non-air hardenable steel currently used in manufacturing is 4130 steel (UNS G10220). This steel does not crack when formed. However, it must be liquid-quenched after heat-treating to attain a high strength and unfortunately this liquid quenching tends to induce high levels of distortion. As a result, liquid quenched materials like 4130 have limitations when used for applications requiring frame-type structures that must be straight and free from distortion. Theoretically, the highest strength-to-weight ratio would be attained if parts of 4130 steel could be assembled together and then heated and liquid quenched as a whole, resulting in a frame with uniformly high-strength throughout all areas. However, liquid quenching an entire frame or large automotive structural component at one time would distort it beyond acceptable limits.
An example of a partially air hardenable steel is 410S (UNS S41008), made available by Allegheny Ludlum of Pittsburgh, PA. 410S is a low carbon modification of 410 (UNS S41000). The low carbon level (0.08% maximum) of 410S prevents austenite formation upon heating, thereby preventing martensite formation upon cooling. This means that the metal doesn't crack during typical forming processes, but it also doesn't harden to a high strength condition. Automotive structural members comprised of 410S would lack the strength needed for load bearing applications.
Additional examples of partially air hardenable steel are True Temper OX Gold and Platinum series, produced by True Temper Sports, Inc. These is a non-stainless steels achieves a high strength without cracking due to the precise addition of expensive alloying components. These alloy steels are specially formulated to mitigate the difficulties inherent in the welding of air hardenable steel. Modifying the material to prevent cracking results in a material too expensive to justify for most structural applications.
As reflected in FIG. 2, air hardenable martensitic stainless steels have exceptionally strength, particularly compared to common metals such as aluminum and even titanium. Nevertheless, even though as shown in FIG. 1 such steels are relatively affordable. Experimentation with air hardenable stainless steel for automotive structural applications appears to have never been attempted due to the paradigm shift in thinking required to produce a high-strength automotive part. Historically, high-strength automotive applications relied on the evolutionary approach of forming ferrous alloys strip, in its final metallurgical microstructure, using successively higher strength steels as the raw material until either the strength targets were met or the part could not be formed due to the material's limitations.
Air hardening steels were first commercially developed for use in cutlery for their high hardness. Common air hardenable steels include martensitic stainless steels. As defined herein, and as understood by those skilled in the art, air hardenable martensitic stainless steels are essentially alloys of chromium and carbon that possess a body-centered-cubic (bcc) or body-centered-tetragonal (bct) crystal (martensitic) structure in the hardened condition. They are ferromagnetic and hardenable by heat treatment, and they are generally mildly corrosion resistant.
Air hardenable martensitic stainless steels include a relatively high carbon and chromium content compared to other stainless steels with a carbon content between 0.08% by weight and 0.75% by weight and a chromium content between 11.5% by weight and 18% by weight. As reflected in FIG. 3, air hardenable martensitic stainless steels have also been defined, and are understood by those skilled in the art, as having a nickel equivalent of between about 4 and 12 and having a chromium equivalent of between about 8 and 15.5, where nickel equivalent is equal to (% Ni+30×% C)+(0.5×% Mn) and chromium equivalent is equal to (% Cr+% Mo+(1.5×% Si)+(0.5×% Nb). Either or both of these definitions are acceptable for practicing the present invention. According to these standard definitions, standard air hardenable martensitic stainless steels include types 403, 410, 414, 416, 416Se, 420, 420F, 422, 431, and 440A-C.
The relatively high carbon and chromium content compared to other stainless steels results in steel with good corrosion resistance, due to the protective chromium oxide layer that forms on the surface, and the ability to harden via heat treatment to a high strength condition. Unfortunately, the high carbon and chromium also presents difficulties related to brittleness and cracking in welding, and accordingly martensitic stainless steel has been primarily used for cutting tools, surgical instruments, valve seats, and shears. Non-stainless air hardenable steels, which contain very high levels of carbon to allow the formation of a martensitic microstructure upon quenching, also present difficulties related to brittleness and cracking.
The use of air hardenable martensitic stainless steels for golf clubs and bicycle applications was introduced in U.S. Pat. No. 5,485,948 and further described in U.S. Pat. No. 5,871,140. These patents describe brazed tube structures that take advantage of the fact that air hardenable stainless steel can be simultaneously brazed and hardened in one heat treating operation. However, there is no suggestion as to how to use such a material for automotive structural members.
This ongoing lack of a strong and lightweight yet low cost automotive structural material is a main hindrance to the development of economically viable low emissions vehicles that can compare in performance, safety, comfort, and price to those powered by the typical internal combustion power system.
Thus, rather than resort to the use of expensive alloys, it would be beneficial to create a process that could utilize common, inexpensive, air hardenable steel to produce automotive structural members substantially free of cracks. Such a process would be even more beneficial if the material possessed the corrosion resistant properties of stainless steel.
Furthermore, it would be desirable for an improved method for manufacturing automotive structural members which are built strong and lightweight, yet are produced at a low costs.